1. Field of Invention
This invention relates to a new and improved solid state valve thermal protection system.
More particularly, the invention relates to such valve thermal protection systems for use with solid state valves employed in high voltage direct current power transmission system converters where direct monitoring of junction temperature is impractical.
2. Prior Art Problem
High voltage direct current (HVDC) power transmission system converter valves are designed with momentary overload capability considerably in excess of their steady state rating. If this momentary overload capability could be utilized safely in the system's operation, the HVDC link could more effectively be used in stabilizing momentary AC system disturbances thereby facilitating system operation. Much of the momentary overload capability could not be utilized if the controls for an HVDC system were designed to limit load current to a worst case steady-state rating which typically might be 1.10 per unit. In order to take advantage of whatever momentary overload capability might be available with any given set of HVDC solid state valves, it is necessary that the controls be designed to allow loads considerably in excess of the steady-state rating; however, it is essential that the duration and amplitude of the overload be limited within a very short time frame when it becomes known that the solid state valve junction temperature is approaching its design limit for maintaining its suppression capability. The present invention makes available a solid state valve thermal protection system for HVDC power converters which enables the solid state valves to be operated in a manner which may take advantage of their momentary overload capability during AC system and other disturbances and which continuously monitors the predicted instantaneous junction temperature of a representative thyristor in said valves so as to immediately signal the need for a reduction in load current should said predicted junction temperature approach its design limit for suppression capability.
The heart of the solid state valve thermal protection system is a solid state valve thermal analog model. This model is in effect an analog computer which is supplied with current input signals representative of the instantaneous heat dissipation within a representative thyristor of the solid state valves, the valve ambient temperature and a signal indicating whether the valves are operating in a blocked or bypass condition (which represents a tripling of the duty on the conducting valves). The solid state valve thermal analog model converts these input signals into a voltage output signal which is continuously analogous to the solid state thyristor junction temperature. This is achieved via circuitry which is designed to simulate the transient thermal resistance characteristics of the solid state thyristor junction and heat sink system to ambient cooling fluid.
U.S. Pat. No. 3,622,849 issued Nov. 23, 1971 to Fred W. Kelley, Jr. of the General Electric Company, discloses a circuit for monitoring the junction temperature of a conducting thyristor by measuring the temperature of a reference point and synthesizing the thermal response of the physical configuration between the thyristor junction and the reference point. The Kelley circuit; however, is designed for use on the AC side of a system and is not effective to measure valve junction temperature during abnormal conditions on the DC side of an HVDC power transmission system such as electronic bypass, commutation failure or blocking. In addition, the Kelley circuit is not designed to include the dynamic effect of ambient temperature changes. Finally, the present invention differs over the Kelley circuit in that it includes a trim capability to compensate for valve dissipation which is not load dependent such as switching, 1/2 LI.sup.2 and other voltage losses, and to compensate for dependence of thermal impedance on quantity of valve cooling fluid flow. U.S. Pat. No. 3,643,130 describes a supervisory device for semiconductor thyristor rectifiers and which employs a thermal replica utilizing an RC component having a variable resistance and capacitor with the variable resistor being comprised by at least one controllable semiconductor resistor. The supervisory device disclosed in U.S. Pat. No. 3,643,130 likewise is designed for use with alternating current systems and includes no automatic means for immediately increasing the thermal replica response upon the onset of an increase in duty cycle such as occurs in an HVDC power transmission system during abnormal conditions such as when the solid state valves of an HVDC power converter are placed in a bypass pair conduction condition. when in bypass pair condition certain of the valves are in a continuous conduction condition as opposed to conduction during only one third of a normal power system cycle in any one valve as occurs during normal operation. Because of the above-discussed characteristics, the prior art devices cannot satisfactorily be used to monitor the performance of the solid state valves of an HVDC power converter where it is desired to operate the HVDC power transmission system in a manner which takes advantage of the momentary overload capability inherently possessed by the valves while maintaining the integrity of the valves'suppression capability, which is the major protection feature of the system.